Re: [Boost-users] boost::any and boost::lexical_cast

From: Chris Ross (cross+boost_at_[hidden])
Date: 2008-06-22 17:57:22

On Jun 22, 2008, at 11:49, Ovanes Markarian wrote:
>> Does anyone have a suggestion as to how I can get a value out of a  
>> boost::any for which I don't know the specific type stored in it?
>  if you know the set of types you operate on consider using
> boost::variant which supports visitor pattern for dispatching of
> currently stored type.

   Yes, I'll have to look into boost::variant.  Thanks for that  
pointer, all.

> Hope that helps. It is difficult to make suggestions, because we don't
> if you are allowed to modify the interface.

   Right.  And a very good point to bring up.  I am constructing this  
100% myself from scratch, so anything *can* be done.

   The super-class in which I'm trying to extract this value is a  
generic database access object.  The boost::any is being returned from  
a "get me the value of column X in table Y" method, implemented by any  
subclass that implements the database-specific layer.  So, the values  
could literally be any value that could be stored in any data value in  
any database, which is why I was trying to use something very generic.

   This particular function, the first I'm implementing in the design  
as I'm building it out, is asking for a value it knows will be  
effectively boolean.  But, depending on the underlying database, may  
be an int, or some other type.  The first case I'm coding is using an  
sqlite3 subclass of the database object, and there is no boolean  
type.  There is an integer type, and since that database (in current  
versions) supports 64-bit integers, I've coded that method to return a  
boost::any<int64_t> when it finds an integer column it's to return.

   I'm not sure boost::variant is sufficiently flexible given what I  
want to do, but it clearly bears investigation as I hadn't looked at  
it yet.  I'd be happy to hear any design suggestions from anyone who  
can think of a good way to do this.  I suppose I could make some sort  
of template class for "database column value", but I'd hoped not to  
have to do that.

                      - Chris